This invention pertains to agents which are useful in medicine as aids in detecting and diagnosing disease, in the examination and evaluation of body organs, and/or for other purposes, and to diagnostic and evaluatory processes using such agents. More particularly, it is concerned with agents which can be labeled with radionuclide tracers for aid in radiological visualization of various types of tissues, including blood, body organs such as heart, liver and lungs, and other organs.
The use of tracer compounds, which emit radiation from within the body, as medical tools has long been known. Early work included the use of such materials for testing liver function and biliary patency, and for the analysis of physiological structure and function, e.g. of the kidneys.
A great deal of information about the body can be obtained by the use of tracer compositions commonly called blood pool agents. Such agents are normally radionuclide-labeled serum albumin, normally radionuclide-labeled human serum albumin for tests in humans, and human or other serum albumin, e.g. bovine serum albumin, for tests in other animals. Such reagents utilize a proteinacious carrier to which a radionuclide such as technetium-99m or iodine-131 has been attached. These agents can be used to obtain a great deal of data, including blood and plasma volume, regional cardiovascular dynamics, global cardiovascular dynamics, e.g. cardiac output, circulation times, protein turnover, placenta localization, brain tumor localization, heart imaging and liver imaging. For example, plasma volume is determined using radionuclide-labeled human serum albumin (HSA), by injecting a known volume of a known concentration of labeled HSA into the subject intravenously. The albumin is substantially confined to the plasma in the blood, and the concentration of the radionuclide-labeled albumin in the plasma is a function of the total plasma volume. By taking blood samples after brief periods, the total fixed volume can be calculated based on the total injected activity and activity measured per unit volume. It is also possible to count the whole blood to determine the whole blood volume with a reasonable degree of accuracy. See Physician's Desk Reference for Radiology and Nuclear Medicine, page 17 (6th ed. 1976). A blood or plasma sample is taken from the patient and counted, and the blood or plasma volume is equal to the radioactive dose originally injected divided by the activity concentration in the sample. The test is fairly reproducible and will give values for given individuals varying within about 5%. The plasma volume is obtained by withdrawing the sample and separating the plasma from the remainder of the whole blood before determining the concentration therein. Whole blood volume is determined by measuring the concentration without separation of the blood components from the sample.
Radionuclide-labeled HSA can also be used for other determinations. For example, cardiac output determinations may be indicated in patients in whom abnormalities of the heart action are detected having borderline heart failure or other myocardial abnormalities. By injecting a small bolus of radionuclide-labeled HSA, for example, and monitoring the ingress of the bolus and the egress of the radioactivity from the heart, the cardiac output can be determined. Other more efficient methods involving gated imaging are also known. Similarly, blood pool agents are useful in diagnosis of pericardial effusion, detection of shunts and other intracardiac abnormalities, differential diagnosis of midline mediastinal masses, diagnosis of ventricular or major vessel aneurysms, and evaluation of patency of major vascular pathways. Both static radiological imaging and radionuclide angiography are used. Use of these techniques to determine pericardial effusion, shunts and other intracardiac abnormalities, midline mediastinal masses, aneurysms, patency of major vascular pathways, and placental localization, particularly for the identification of placenta previa, are shown in PDR for Radiology, supra, at pages 44-46.
Other similar materials have been used with better advantage in other parts of the body. For example, denatured, macroaggregated HSA tagged with radionuclide, has been used to advantage in pulmonary perfusion studies in the lungs. Labeled microspheres of denatured albumin have also been used for that purpose.
Blood pool agents have also been utilized to image the liver, particularly the vascular compartment thereof. Also, microaggregates, having a particle size of less than 5.mu.m, preferably 0.1 to 5.mu.m, has been used to particular advantage in liver studies. Microaggregated serum albumin will be referred to as MIA, so as to distinguish between it and macroaggregated albumin (MAA).
Substantially all of the known uses of HSA have suffered from extremely difficult problems, primarily caused by the lack of stability of products containing standard human albumin, particularly at low pH. For example, preparation of stannous albumin compositions using the pH's necessary to solubilize stannous ions. Standard human serum albumin results in a product which turns cloudly and quickly precipitates out of solution. Even if it is freeze-dried (lyophilized) shortly after preparation, when it is reconstituted, turbidity forms within an hour after reconstitution. This is a serious disadvantage, particularly when the HSA is provided as part of a kit for conducting the full radiological test, since, as previously known, the HSA is so unstable that the same kit cannot be used to cover the work for a single day, i.e. the HSA clouds up and precipates are found. Yet low pH's, e.g. around 3, are necessary to ensure high uptake of the labeled HSA complex in the blood. Moreover, the effective life of some known HSA diagnostic agents is so short that they must normally be used before they can be effectively tested to ensure non-pyrogenicity and sterility thereof.
Still further problems and complications are encountered in the preparation of radioactive-labeled microaggregated and macroaggregated human serum albumin because of the difficulty in controlling the aggregation procedure so as to attain the desired particle size consistent with the intended diagnostic use of the material. Microaggregated HSA, having a particle size generally within the range of 0.1 to 5.mu.m, is primarily used in imaging liver and spleen. Macroaggregated HSA, having a particle size greater than 5.mu.m, preferably less than 100.mu.m, most preferably 15 to 50.mu.m, is an excellent agent for visualizing the lungs. These materials are selectively collected in the organs that they are used to diagnose, and they essentially collect only in those portions of the organs having sufficient blood supply, thus permitting effective visualization of areas having good and impeded blood supply. These proteinacious materials eventually dissolve, and thus do not prevent blood from reaching the areas in which they are located for significant periods.
It is, accordingly, an object of the present invention to provide a method by which serum albumin may be stabilized for use over wide pH ranges. It is a further object to provide radiological agents which are highly suited for use in vivo, giving maximal information while at the same time exposing the body to minimal radiation dosage. It is a further object of the invention to provide a composition comprising reducing agent with stabilized serum albumin, suitable for complexing or tagging with radionuclides for radioactive scanning. A still further object of the present invention is to provide radionuclide-tagged serum albumin compounds or complexes which remain soluble at low pH's and can be used to advantage for long periods for radioactive scanning. Further objects of the invention include the provision of kits suitable for carrying out the full testing procedures for radiological tests, e.g. blood volume, plasma volume, metabolism and turnover studies, analyses for pulmonary emboli, bronchogenic carcinoma, pneumonitis, pulmonary emphysema, chronic pulmonary tuberculosis, pulmonary vascular obliteration, neoplasm, pulmonary ischemia or infarction, pulmonary circulation or other disorders, brain tumor localization, cisternography, blood flow studies, cardiovascular dynamics, including cardiac output, cardiac blood volume, circulation times, protein turnover, placenta localization, brain and heart imaging, liver, spleen and bone marrow imaging, and location of growths or absesses, or for other uses.
Further objects and advantages of the present invention will be readily apparent to the skilled in the art from a consideration of the present disclosure or from practice of the invention disclosed herein.